A discharge lamp having a high mercury vapor pressure is used for a light source of a projector device. In such a high-voltage mercury lamp, the light in a visible wavelength region can be obtained with a high output by making the mercury vapor pressure high.
The discharge lamp has a light emitting part which is formed by a discharge vessel and has a substantially spherical shape, and a pair of electrodes is arranged in the light emitting part so as to be opposite to each other with an extremely small distance, for example, 2 mm or less.
In the case that the discharge lamp described above is lighted in the same state for a long period, a plurality of minute protrusions may be formed due to a high temperature, or minute irregularities may be generated in the tip surface of the electrodes. The minute protrusions and the irregularities are generated by the melting of a material (for example, tungsten) constituting the electrodes and the aggregation of a chemical compound generated by being bonded to gas which is sealed within the light emitting part, and this existence changes the shape of the surface of the tip of the electrode. It has been known that a starting point of an arc moves in conjunction with this, the discharge position becomes unstable, and there has been a problem that a flickering of a projection light called as a flicker is generated.
In order to solve such a problem, Patent Document 1 discloses an illumination system of a discharge lamp which supplies a current pulse P1 having a predetermined frequency (a fundamental frequency) to a discharge lamp and which intermittently or periodically inserts a current pulse P2 having a lower frequency than the fundamental frequency to the current pulse P1 (refer to FIG. 9).
A period that one electrode is fixed to an anode and the other electrode is fixed to a cathode, that is, a period that a high voltage is applied between both the electrodes becomes longer by setting the frequency of the current pulse to the low frequency. As a result, a heating degree applied to the electrode is enhanced, and it is possible to transmit the heat not only to the tip of the electrode, but also to a position which is away from the tip. Therefore, the heat is transmitted to the position which is away from the tip of the electrode during the application of the current pulse having the low frequency, and it is possible to melt and evaporate the minute protrusions or the irregularities which are generated at the positions. As a result, it is possible to eliminate the protrusions and the irregularities in the other positions than the tip of the electrode which may adversely affect, and it is possible to stabilize a luminescent spot of the arc.
Meanwhile, the constant power lighting control is generally carried out by the power supply part (the power supply device) in the discharge lamp. This control is carried out for the purpose of stably keeping a light output from the discharge lamp and for the purpose of fixedly keeping a load applied to the discharge lamp, in more detail, a thermal load applied to a discharge vessel which is constructed by a quartz glass.
In the discharge lamp, the electrode is consumed with an elapse of the lighting time, and the distance between the electrodes is elongated. When the distance between the electrodes is elongated, the voltage (the lamp voltage) applied to the discharge lamp comes up. As a result, the current (the lamp current) applied to the discharge lamp is lowered under the constant power lighting control. When the lamp current is lowered, a thermal inflow to the electrode is reduced. Therefore, even if the pulse P2 having the low frequency is supplied to the discharge lamp under this state, an amount of heat flowing into the electrode is reduced, and the effect of eliminating the protrusions or the irregularities in the other positions than the tip of the electrode described above is lowered.
In view of this point, Patent Document 2 discloses an illumination system of a discharge lamp which detects a change of a lamp voltage and controls to further lower a frequency of a current pulse having a low frequency (make a pulse width long) in the case that the lamp voltage comes up, that is, in the case that the lamp current comes down.